1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens. The present invention is suitable for, for example, imaging systems for digital cameras, video cameras, and silver-halide film cameras.
2. Description of the Related Art
A negative-lead zoom lens is known in which a lens unit having a negative refractive power is disposed at a position closest to the object side. The negative-lead zoom lens has a relatively small close-up shooting distance, and the field angle and can be relatively easily increased. In addition, the back focus can be easily increased. Therefore, the negative-lead zoom lens is often used as a wide field angle imaging lens.
A zoom lens including at least four lens units, which are a lens unit having a negative refractive power, a lens unit having a positive refractive power, a lens unit having a negative refractive power, and a lens unit having a positive refractive power arranged in order from the object side, is known as a negative-lead zoom lens (U.S. Pat. Nos. 5,517,361, 7,184,221, and U.S. Pat. No. 5,710,669).
U.S. Pat. No. 5,517,361 describes a zoom lens including a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, a third lens unit having a negative refractive power, and a fourth lens unit having a positive refractive power, which are arranged in order from the object side to the image side. The zoom lens performs zooming by changing the distances between the lens units and performs focusing by moving the third lens unit and the fourth lens unit.
U.S. Pat. No. 7,184,221 describes a zoom lens including a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, a third lens unit having a negative refractive power, a fourth lens unit having a positive refractive power, a fifth lens unit having a negative refractive power, and a sixth lens unit having a positive refractive power, which are arranged in order from the object side to the image side. The zoom lens performs zooming by changing the distances between the lens units and performs focusing by moving the fifth lens unit.
U.S. Pat. No. 5,710,669 describes a zoom lens including four lens units, which are a lens unit having a negative refractive power, a lens unit having a positive refractive power, a lens unit having a negative refractive power, and a lens unit having a positive refractive power, which are arranged in order from the object side to the image side. The zoom lens performs zooming by changing the distances between the lens units. In the zoom lens, the first lens unit is divided into a front unit having a negative refractive power and a rear unit having a negative refractive power, and focusing is performed by moving the rear unit.
U.S. Pat. No. 5,576,890 describes a zoom lens including a first lens unit having a negative refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a negative refractive power, and a fifth lens unit having a positive refractive power, which are arranged in order from the object side to the image side. This zoom lens performs zooming by changing the distances between the lens units and performs focusing by moving the first lens unit.
Recently, there has been an increasing demand for zoom lenses for use in digital cameras in which the size of the entire lens system is small, which have a wide imaging field angle, and with which high-quality images can be captured.
In particular, chromatic aberrations, which affect the degree of color bleed or the resolution of images when the light source is a white light source, are required to be adequately corrected over the entire zooming range from the wide-angle end to the telephoto end and over the entire object distance range from infinity to a close distance.
In a negative-lead zoom lens, aberrations, in particular the chromatic aberrations, largely vary when the object distance varies.
In particular, in the negative-lead zoom lens, when the zoom ratio is increased to increase the shooting area, the chromatic aberration of magnification increases at the wide-angle end and both the chromatic aberration of magnification and the axial chromatic aberration increase at the telephoto end.
Even if the chromatic aberrations are sufficiently corrected when the object distance is infinity, the chromatic aberrations increase when the object distance is changed to a close distance. Therefore, to obtain high-quality images over the entire shooting area with a negative-lead zoom lens, it is important to reduce the variation in chromatic aberrations during focusing.
More specifically, in a high-zoom-ratio zoom lens having a normal focal length, that is, a focal length close to the dimension of an image circle (effective screen), the chromatic aberration of magnification generally occurs in the positive direction at the wide-angle end. In other words, light with an ultra-long wavelength and light with an ultra-short wavelength tend to focus at a higher image height compared to the focus position for the design wavelength.
Conversely, the chromatic aberration of magnification occurs in the negative direction at the telephoto end. In other words, light with an ultra-long wavelength and light with an ultra-short wavelength tend to focus at a lower image height compared to the focus position for the design wavelength.
In general, a zoom lens performs focusing by moving one or more of lens units included in the entire lens system along the optical direction.
In this case, the focal length of the entire lens system in the state in which an object at infinity is in focus differs from the focal length of the entire lens system in the state in which an object at a close distance is in focus.
If the focal length in the state in which an object at a close distance is in focus is smaller than the focal length in the state in which an object at infinity is in focus, the chromatic aberration of magnification for the close distance occurs in the positive direction compared to the chromatic aberration of magnification for infinity. In other words, light with an ultra-short wavelength focuses at a higher image height compared to the focus position for the design wavelength.
Conversely, if the focal length in the state in which an object at a close distance is in focus is larger than the focal length in the state in which an object at infinity is in focus, the chromatic aberration of magnification for the close distance occurs in the negative direction compared to the chromatic aberration of magnification for infinity. In other words, light with an ultra-short wavelength focuses at a lower image height compared to the focus position for the design wavelength.
To reduce the variations in aberrations during focusing, a so-called floating method is used in which a plurality of lens units are independently moved.
The floating method is effective in reducing the variations in aberrations during focusing. However, in a negative-lead zoom lens, since the lens structure is significantly asymmetric, it is difficult to adequately correct the chromatic aberrations caused when the object distance changes even when the floating method is used. Thus, it is extremely difficult to obtain high optical performance over the entire object distance range.